A known internal combustion engine for an outboard motor disclosed in, for example, Japanese Utility Model Registration No. 2,500,711, is provided with a carburetor including a throttle body provided with an intake passage having a substantially horizontal axis, and a butterfly-type throttle valve placed in the intake passage.
Referring to FIG. 5 showing this known internal combustion engine provided with a carburetor a having a low-speed fuel system) s, intake air flows through a narrow gap f between the circumferential edge c of a throttle valve b and a side wall e defining an intake air passage d into a combustion chamber while the internal combustion engine is idling. The internal combustion engine operates at a low engine speed during idling. Therefore, valve overlap time in which both the intake valve and the exhaust valve are open is comparatively long during idling and hence, spitting, i.e., the flow of the exhaust gas into the intake passage, occurs in some cases. If the exhaust gas spat back into the intake passage contains foreign matters, such as lubricating oil and combustion products including carbon, the foreign matters deposit on the circumferential edge of the throttle valve b or on the side wall e in a solid matter when the spat back exhaust gas flows through the gap f. The deposited solid matter exerts resistance against the flow of air flowing through the gap f to reduce the flow rate of air and, consequently, the idling speed of the internal combustion engine decreases.
The outboard motor, in particular, is operated for a long time in an idling mode in which the throttle valve is fully closed or in a very-light-load operating mode in which the throttle valve is slightly opened for trolling. Since the outboard motor is operated frequently in such a light-load operating mode, the solid matter is likely to deposit in the gap f. In some cases, the deposited solid matter makes the engine speed unstable while the internal combustion engine is operating in an idling mode or a very-light-load mode. The maintenance of the internal combustion engine needs to be performed at short intervals to avoid such unstable operation of the internal combustion engine due to the deposited solid matter.